The present invention relates to a convergent charged particle beam apparatus using a charged particle beam such as an electron beam or ion beam for microstructure fabrication or observation and an inspection method using the same, and more particularly to an automatic focusing system and arrangement in the convergent charged particle beam apparatus.
As an example of an apparatus using a charged particle beam, there is an automatic inspection system intended for inspecting and measuring a microcircuit pattern formed on a substrate such as a semiconductor wafer. In defect inspection of a microcircuit pattern formed on a semiconductor wafer or the like, the microcircuit pattern under test is compared with a verified non-defective pattern or any corresponding pattern on the wafer under inspection. A variety of optical micrograph imaging instruments have been put to practical use for this purpose, and also electron micrograph imaging has found progressive applications to defect inspection by pattern image comparison. In a scanning electron microscope instrument which is specifically designed for critical-dimension measurement of line widths and hole diameters on microcircuit patterns used for setting and monitoring process conditions of semiconductor device fabrication equipment, automatic critical-dimension measurement is implemented through use of image processing.
In comparison inspection where electron beam images of corresponding microcircuit patterns are compared for detecting a possible defect or in critical-dimension measurement where electron beam images are processed for measuring such dimensions as pattern line widths, reliability of results of inspection or measurement largely depends on the quality of electron beam images. Deterioration in electron beam image quality occurs due to image distortion caused by deflection or aberration in electron optics, decreased resolution caused by defocusing, etc., resulting in degradation of performance in comparison inspection or critical-dimension measurement.
In a situation where a specimen surface is not uniform in height, if inspection is conducted on the entire surface area under the same condition, an electron beam image varies with each region inspected as exemplified in FIGS. 1(a)-1(d), wherein FIG. 1(a) shows a wafer with different regions A-C, FIG. 1(b) shows an in-focus image of region A and FIGS. 1(c) and 1(d) show defocused images of regions B and C, respectively. In inspection by comparison between the in-focus image of FIG. 1(b) and the defocused image FIG. 1(c) or FIG. 1(d), it is impossible to attain correct results. Further, since these images provide variation in pattern dimensions and results of edge detection on them are unstable, pattern line widths and hole diameters cannot be measured accurately. Conventionally, image focusing on an electron microscope is performed by adjusting a control current to an objective lens thereof while observing an electron beam image. This procedure requires a substantial amount of time and involves repetitive scanning on a surface of a specimen, which may cause a possible problem of specimen damage.
In Japanese Non-examined Patent Publication No. 258703/1993, there is disclosed a method intended for circumventing the abovementioned disadvantages, wherein an optimum control current to an objective lens for each surface height of a specimen is pre-measured at some points on the specimen and then, at the time of inspection, focus adjustment at each point is made by interpolation of pre-measured data. However, this method is also disadvantageous in that a considerable amount of time is required for measuring an optimum objective lens control current before inspection and each specimen surface height may vary during inspection depending on wafer holding conditions.
A focus adjustment method for a scanning electron microscope using an optical height detecting arrangement is found in Japanese Non-examined Patent Publication No. 254649/1988. However, since an optical element for height detection is disposed in a vacuum system, it is rather difficult to perform optical axis alignment.
In microstructure fabricating equipment using a convergent charged particle beam, focus adjustment of the charged particle beam has a significant effect on fabrication accuracy, i.e., focus adjustment is of extreme importance as in instruments designed for observation. Examples of microstructure fabricating equipment include an electron beam exposure system for forming semiconductor circuit patterns, a focused ion beam (FIB) system for repairing circuit patterns, etc.
In a scanning electron microscope, a method of measuring an optimum control current to an objective lens thereof through electron beam imaging necessitates attaining a plurality of electron beam images for detecting a focal point, thus requiring a considerable amount of time for focus adjustment. That is, such a method is not suitable for focusing in a short time. Further, in an application of automatic inspection or critical-dimension measurement over a wide range, focus adjustment at every point using the abovementioned method is not practicable, and it is therefore required to perform pre-measurement at some points before inspection and then estimate a height at each point through interpolation, for instance. FIG. 2 shows an overview of an electron-beam automatic semiconductor device inspection system to which the present invention is directed. In such an automatic inspection system, a specimen wafer under inspection is moved by means of stages with respect to an electron optical system thereof for carrying out wide-range inspection.
A semiconductor wafer to be inspected in a fabrication process may deform due to heat treatment or other processing, and a degree of deformation will be on the order of some hundreds of micrometers in the worst case. However, it is extremely difficult to hold the specimen wafer stably without causing interference with electron optics in a vacuum specimen chamber, and also it is impossible to adjust specimen leveling as in an optical inspection system using vacuum chucking.
Further, since a substantial amount of time is required for inspection, a specimen holding state may vary due to acceleration/deceleration in reciprocating stage movement, thereby resulting in a specimen surface height being different from a pre-measured level.
For the reasons mentioned above, there is a rather high degree of possibility that a surface height of a specimen under inspection will vary unstably exceeding a focal depth of the electron optical system (a depth of focus is generally on the order of micrometers at a magnification of 100×, but that necessary for semiconductor device inspection depends on inspection performance requirements concerned). For focus adjustment using electron beam images, a plurality of electron images must be attained at each point of interest with each stage being stopped. It is impossible to conduct focus adjustment continuously while detecting a height at each point simultaneously with stage movement for the specimen under inspection.
In an approach that focus adjustment using electron beam images is performed at some points on a specimen surface before the start of inspection, an amount of time is required for calibration before inspection. This causes a significant decrease in throughput as a size of wafer becomes larger. Since there is a technological trend toward larger-diameter wafers, a degree of wafer deformation such as bowing or warping will tend to be larger, resulting in more stringent requirements being imposed on automatic focusing functionality. Depending on the material of a specimen, exposure with an electron beam may alter an electric charge state on specimen surface to cause an adverse effect on electron beam images used for inspection.
In consideration of the above, it is difficult to ensure satisfactory performance in long-period inspection on a scanning electron microscope instrument using the conventional methods. Where stable holding of a specimen is rather difficult, it is desirable to carry out specimen surface height detection in a range of electron optical observation immediately before images are attained during inspection. Further, where inspection is conducted while each stage is moved continuously, specimen surface height detection must also be carried out continuously at high speed without interrupting a flow of inspection operation. For realizing continuous surface height detection simultaneously with inspection, it is required to detect a height of each inspection position or its vicinity at high speed.
However, if any element which affects an electric or magnetic field, e.g., an insulating or magnetic element, is disposed in the vicinity of an observation region, electron beam scanning is affected adversely. It is therefore impracticable to mount a sensor in the vicinity of electron optics. Further, since the observation region is located in the vacuum specimen chamber, measurement must be enabled in a vacuum. For use in the vacuum specimen chamber, it is also desirable to make easy adjustment and maintenance available. While there have been described conditions as to an example of an electron-beam inspection system, these conditions are also the same in a microstructure observation/fabrication system using an ion beam or any other convergent charged particle beam. Further, since there are the same conditions in such systems that images of an aperture, mask, etc. are formed or projected as well as in a system where a charged particle beam is converged into a single point, it is apparent that the present invention is applicable to charged particle beam systems comprising any charged particle beam optics for image formation/projection.